Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 26
In Figure 5-1, Notebook B sends a transmission to Notebook A. The packet takes these steps:
1. From ME-S (B) to the local MDBS.
2. MDBS to the controlling MD-IS (A) where the destination user is determined to be at home.
3. MD-IS (A) back to the local MDBS.
4. MDBS to ME-S (A); the destination.
M-ES (A)
MDBS MD-IS (A)
IS (Router)
M-ES (B)
MD-IS (B)
Internet
4
3
2
1
Figure 5-1: CDPD mobility management scenario 1
The packet travels a short path because both the sender and receiver are within the home
subnetwork of the destination Notebook A. The path passes over the airlink (1) to MD-IS (A) (2)
and directly back out to Notebook A, again over the airlink (3 and 4).
In scenario 2 (Figure 5-2 on the following page), Notebook A sends a reply transmission to
Notebook B. In this case, the packet travels through a much longer path because Notebook B is
roaming. The steps are:
1. From ME-S (A) to the local MDBS.
2. MDBS to the controlling MD-IS (A) where the destination user (NEI) is determined to be on
the Internet.
3. MD-IS (A) routes through the gateway IS.
4. The IS sends the packet to the Internet (and any number of IS routers).
5. The Internet eventually delivers the packet to the home of the destination NEI, MD-IS (B).
6. MD-IS (B) knows that the user is roaming on MD-IS (A) and routes the packet back there
using the TEI.
7. MD-IS (A) now knows the true destination (TEI) is on the local MDBS.
8. MDBS to the destination ME-S (B).